Chapter 21, Problem 34A

Interpretation Introduction

Interpretation: The similarity and difference between dry cells and alkaline batteries need to be explained.

Concept Introduction: A dry cell is a type of voltaic cell in which electrolysis is in the form of a paste. Here, oxidation takes place at the anode and reduction occurs at the cathode. An alkaline battery is an improved dry cell. Here, reactions are similar to common dry cells but the electrolyte is a basic KOH paste in an alkaline battery.

Explanation of Solution

Similarity between dry cells and alkaline batteries:

Both dry cells and alkaline batteries are types of voltaic cells. Here, oxidation occurs at the anode and reduction at the cathode. In both dry cell and alkaline batteries, the zinc container acts as an anode, and the graphite rod as the cathode. In both cases, the graphite rod act as a conductor and does not undergo reduction even though it is a cathode. Here, the reduction of manganese (IV) oxide occurs at the cathode.

Differences between dry cells and alkaline batteries:

In a dry cell, a zinc container is filled with a paste (electrolyte) of manganese (IV) oxide or ${\text{MnO}}_{\text{2}}$ , ammonium chloride or ${\text{NH}}_{\text{4}}\text{Cl}$ , zinc chloride or ${\text{ZnCl}}_{\text{2}}$ and water or ${\text{H}}_{\text{2}}\text{O}$ . On the other hand, in an alkaline battery, a basic paste of manganese (IV) and KOH is taken as an electrolyte. In a dry cell, ammonia gas is released during reduction. Here, the reduction of manganese (IV) oxide occurs at the cathode. The reaction is represented as follows:

  ${\text{2MnO}}_{\text{2}}\left(s\right)+{\text{2NH}}_4^{+}\left(aq\right)+2e\to {\text{Mn}}_{\text{2}}{\text{O}}_{\text{3}}\left(s\right)+2{\text{NH}}_{\text{3}}+{\text{H}}_{\text{2}}\text{O}\left(l\right)$

Ammonia gas is not built up in the case of an alkaline battery. Here, the zinc electrode is also maintained as it corrodes slowly under basic conditions.